Infrared Fixed-Focus Lens

ABSTRACT

The invention provides an infrared fixed-focus lens of which component lens pieces are made of germanium featured by low chromatic dispersion, and have no surface processed to serve as diffraction optics. The infrared fixed-focus lens comprises the first lens piece disposed closer to an object and of negative refractivity, and the second lens piece disposed closer to the image plane and of positive refractivity. Both the first and second lens pieces are made of germanium.

FIELD OF THE INVENTION

The present invention relates to an infrared fixed-focus lens, and moreparticularly, to an infrared fixed-focus lens adopted to suppressspherical aberration at the wide-angle end and suitable to infraredthermography optical systems and surveillance cameras. The term‘infrared’ used herein means radiations including middle infrared raysof wavelength ranging from 3000 to 5000 nm and far infrared rays ofwavelength ranging from 8000 to 14000 nm.

BACKGROUND OF THE INVENTION

As an example of the prior art infrared lenses capable of producingexcellent images and sturdy enough to endure severe environments, aninfrared optical system suitable for use in surveillance cameras hasbeen proposed which is compatible with infrared rays through farinfrared rays, namely, with a wavelength range from 3 μm to 14 μm, andis of dual-lens configuration where the first lens disposed closer to anobject is a convex meniscus lens having its convex surface faced to theobject while the second lens disposed closer to the image plane isanother convex meniscus lens having its concave surface faced to theobject, and at least one of the first and second lenses has its oppositesurfaces processed to serve as diffraction optics (See Patent Document 1or Official Gazette of JP-A-2010-113191).

The infrared optical system disclosed in Patent Document 1 issubstantially inappropriate to use for a wide-angle lens since its firstlens is the convex meniscus lens. In embodiments in Patent Document 1,all the lenses are made of chalcogenide. Chalcogenide is low indiffractive index and great in chromatic dispersion, and hence, in orderto compensate for chromatic aberration, the lens must have itssurface(s) processed to serve as diffraction optics. In Patent Document1, all the embodiments have their respective lens surfaces processed tobe diffraction optics.

The present invention is made to overcome the aforementioneddisadvantages of the prior art infrared lenses, and accordingly, it isan object of the present invention to provide an infrared fixed-focuslens that is of wide-angle, is made of germanium exhibiting a lowchromatic dispersion, and includes no lens pieces with a surface servingas diffraction optics.

SUMMARY OF THE INVENTION

The present invention provides an infrared fixed-focus lens thatcomprises the first lens piece closer to an object and of negativerefractivity and the second lens piece closer to the image plane and ofpositive refractivity, and that attains the full field angle of 24 to 55degrees.

Although it is of dual-lens configuration, the infrared fixed-focus lensaccording to the present invention has the first or foremost lens pieceprocessed to show negative refractivity, and hence, the lens as a wholecan satisfactorily compensate for comatic aberration and distortionwhile, simultaneously, the second lens piece of positive refractivity isable to satisfactorily compensate for spherical aberration developed inthe first lens piece of negative refractivity.

Various aspects of the present invention will be described below.

<1st Aspect of the Invention>

In the infrared fixed-focus lens in one aspect of the invention, thefirst and second lens pieces are made of germanium. Germanium featuredby high refractive index and low chromatic dispersion enablescompensation for chromatic aberration without any lens surface processedto serve as diffraction optics.

<2nd Aspect of the Invention>

In the infrared fixed-focus lens in another aspect of the invention, thefirst lens piece having its front surface closer to the object shaped inconvexity exhibits negative refractive power while the second lens piecehaving its rear surface closer to the image plane shaped in convexityexhibits positive refractive power.

The infrared fixed-focus lens in accordance with the present invention,although of dual-lens configuration, has the first lens piece processedto show negative refractivity, and hence, the lens as a whole cansatisfactorily compensate for comatic aberration and distortion while,simultaneously, the second lens piece of positive refractivity is ableto satisfactorily compensate for spherical aberration developed in thefirst lens piece of negative refractivity.

<3rd Aspect of the Invention>

The infrared fixed-focus lens in still another aspect of the inventionmeets the requirements as defined in the following formulae (1):

−4.5≦f1/f≦−1.55  (1)

where f1 is a focal length of the first lens piece, and f is a focallength of the entire optics.

The formulae (1) provide conditions to suppress field curvature. If theterm or the ratio f1/f is smaller or greater to go beyond the lower orupper limit defined in the formulae, it becomes hard to correct thefield curvature.

<4th Aspect of the Invention>

The infrared fixed-focus lens in further another aspect of the presentinvention meets the requirements as defined in the following formulae(2):

0.6≦d/f≦1.9  (2)

where d is a distance from the first lens piece to the second lenspiece.

The formulae (2) provide conditions in which the second lens piece has adiameter not too large, and the lens as a whole has a back focussufficiently long. If the term or the ratio d/f exceeds the upper limitdefined in the formulae (2), the second lens piece has a diameterexcessively large. If d/f is smaller to go beyond the lower limit, thelens as a whole cannot obtain a back focus sufficiently long.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a first embodiment of an infraredfixed-focus lens according to the present invention;

FIG. 2 depicts a graph of spherical aberration developed in the firstembodiment of the infrared fixed-focus lens;

FIG. 3 depicts graphs of astigmatism developed in the first embodimentof the infrared fixed-focus lens;

FIG. 4 depicts a graph of distortion developed in the first embodimentof the infrared fixed-focus lens;

FIG. 5 is a sectional view showing a second embodiment of the infraredfixed-focus lens according to the present invention;

FIG. 6 depicts a graph of spherical aberration developed in the secondembodiment of the infrared fixed-focus lens;

FIG. 7 depicts graphs of astigmatism developed in the second embodimentof the infrared fixed-focus lens;

FIG. 8 depicts a graph of distortion developed in the second embodimentof the infrared fixed-focus lens;

FIG. 9 is a sectional view showing a third embodiment of the infraredfixed-focus lens according to the present invention;

FIG. 10 depicts a graph of spherical aberration developed in the thirdembodiment of the infrared fixed-focus lens;

FIG. 11 depicts graphs of astigmatism developed in the third embodimentof the infrared fixed-focus lens;

FIG. 12 depicts a graph of distortion developed in the third embodimentof the infrared fixed-focus lens;

FIG. 13 is a sectional view showing a fourth embodiment of the infraredfixed-focus lens according to the present invention;

FIG. 14 depicts a graph of spherical aberration developed in the fourthembodiment of the infrared fixed-focus lens;

FIG. 15 depicts graphs of astigmatism developed in the fourth embodimentof the infrared fixed-focus lens;

FIG. 16 depicts a graph of distortion developed in the fourth embodimentof the infrared fixed-focus lens;

FIG. 17 is a sectional view showing a fifth embodiment of the infraredfixed-focus lens according to the present invention;

FIG. 18 depicts a graph of spherical aberration developed in the fifthembodiment of the infrared fixed-focus lens;

FIG. 19 depicts graphs of astigmatism developed in the fifth embodimentof the infrared fixed-focus lens;

FIG. 20 depicts a graph of distortion developed in the fifth embodimentof the infrared fixed-focus lens;

FIG. 21 is a sectional view showing a sixth embodiment of the infraredfixed-focus lens according to the present invention;

FIG. 22 depicts a graph of spherical aberration developed in the sixthembodiment of the infrared fixed-focus lens;

FIG. 23 depicts graphs of astigmatism developed in the sixth embodimentof the infrared fixed-focus lens;

FIG. 24 depicts a graph of distortion developed in the sixth embodimentof the infrared fixed-focus lens;

FIG. 25 is a sectional view showing a seventh embodiment of the infraredfixed-focus lens according to the present invention;

FIG. 26 depicts a graph of spherical aberration developed in the seventhembodiment of the infrared fixed-focus lens;

FIG. 27 depicts graphs of astigmatism developed in the seventhembodiment of the infrared fixed-focus lens;

FIG. 28 depicts a graph of distortion developed in the seventhembodiment of the infrared fixed-focus lens;

FIG. 29 is a sectional view showing an eighth embodiment of the infraredfixed-focus lens according to the present invention;

FIG. 30 depicts a graph of spherical aberration developed in the eighthembodiment of the infrared fixed-focus lens;

FIG. 31 depicts graphs of astigmatism developed in the eighth embodimentof the infrared fixed-focus lens; and

FIG. 32 depicts a graph of distortion developed in the eighth embodimentof the infrared fixed-focus lens.

EMBODIMENTS OF THE PRESENT INVENTION

Detailed below will be data of each of the embodiments of the infraredfixed-focus lens in accordance with the present invention. All of theexemplary infrared fixed-focus lenses are identical in wavelength of 10μm.

Embodiment 1

Focal Length 8.4 mm F num. F/1.0 Angle of Field 2ω = 50° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 28.1198 2.5000 Germanium 2 (ASPH) 18.48482.0002 3 (STOP) 12.0905 4 (ASPH) −366.6150 6.0000 Germanium 5 (ASPH)−28.1404 13.9190

Aspheric surfaces can be expressed as in the following formula (3):

$\begin{matrix}{X = {\frac{H^{2}/R}{1 + \sqrt{1 - \left( {ɛ\; {H^{2}/R^{2}}} \right)}} + {AH}^{2} + {BH}^{4} + {CH}^{6} + {DH}^{8} + {EH}^{10}}} & (3)\end{matrix}$

where X is an aspherized shape, R is a curvature of radius, ε is a conicconstant, and H is a height from the optical axis (in millimeters).

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 1.0000 0.0000E+004.8636E−04 −2.0284E−06 2.7596E−08   2.9694E−10 2 1.0000 0.0000E+007.7063E−04   8.9604E−06 −2.9221E−07     1.7511E−08 4 1.0000 0.0000E+00−1.4004E−05   −9.9035E−08 8.7376E−10 −2.4866E−12 5 1.0000 0.0000E+008.0977E−06 −9.9537E−08 5.9356E−10 −1.4124E−12

The value related to formulae (1) is given as follows: f1/f=−2.650

The value related to formulae (2) is determined as follows: d/f=1.677

Embodiment 2

Focal Length 8.34 mm F num. F/1.0 Angle of Field 2ω = 49.66° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 15.0696 2.5000 Germanium 2 (ASPH) 11.49992.0002 3 (STOP) 11.2654 4 (ASPH) 331.4916 6.0000 Germanium 5 (ASPH)−28.1834 10.7824

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 1.0000 0.0000E+003.5965E−04 3.8914E−06 −1.0751E−07     2.6374E−09 2 1.0000 0.0000E+007.2485E−04 2.6149E−05 −1.2666E−06     6.2477E−08 4 1.0000 0.0000E+00−2.5357E−05   −9.5073E−08   8.5019E−10 −2.0664E−12 5 1.0000 0.0000E+008.3342E−06 −1.2120E−07   6.1565E−10 −1.2373E−12

The value related to formulae (1) is given as follows: f1/f=−4.070

The value related to formulae (2) is determined as follows: d/f=1.590

Embodiment 3

Focal Length 8.40 mm F num. F/1.0 Angle of Field 2ω = 50.48° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 47.5084 2.5000 Germanium 2 (ASPH) 20.81831.5000 3 (STOP) 14.2683 4 (ASPH) −104.8620 6.0000 Germanium 5 (ASPH)−27.6387 19.7897

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 1.0000 0.0000E+006.0412E−04 −5.2552E−06 3.9653E−08 2.9914E−10 2 1.0000 0.0000E+008.0092E−04   1.8934E−05 −1.0322E−06   2.8721E−08 4 1.0000 0.0000E+00−6.6122E−06   −1.0390E−07 8.7916E−10 −2.5811E−12   5 1.0000 0.0000E+007.5210E−06 −9.1623E−08 5.8044E−10 −1.3942E−12  

The value related to formulae (1) is given as follows: f1/f=−1.580

The value related to formulae (2) is determined as follows: d/f=1.877

Embodiment 4

Focal Length 11.6 mm F num. F/1.0 Angle of Field 2ω = 35.2° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 18.1488 2.0000 Germanium 2 (ASPH) 13.361518.9283 3 (STOP) 0.5000 4 (ASPH) 512.5988 6.5000 Germanium 5 (ASPH)−45.1966 20.4043

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 −13.8410 0.0000E+003.1570E−04 −3.0495E−06 2.3921E−08 −1.0937E−10 2 −8.5596 0.0000E+005.2940E−04 −4.6784E−06 4.1185E−08 −1.7221E−10 4 −299.0000 0.0000E+00−1.8928E−05   −6.9160E−09 −6.3805E−11   −1.6919E−12 5 −0.0565 0.0000E+001.0974E−05 −2.0749E−08 8.0414E−11 −1.0684E−12

The value related to formulae (1) is given as follows: f1/f=−2.110

The value related to formulae (2) is determined as follows: d/f=1.675

Embodiment 5

Focal Length 13.0 mm F num. F/1.0 Angle of Field 2ω = 33.7° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 20.0889 2.0000 Germanium 2 (ASPH) 14.26943.0212 3 (STOP) 11.2274 4 (ASPH) −96.5908 9.0000 Germanium 5 (ASPH)−31.3032 23.1695

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 −14.223 0.0000E+00  2.9998E−04 −2.7714E−06 1.0555E−08 −3.4876E−11 2 −7.2342 0.0000E+00  4.9252E−04 −3.4672E−06 9.4283E−09 −3.8809E−11 4 −19.2590 0.0000E+00−1.6900E−05 −3.1649E−08 2.0797E−10 −3.9529E−13 5 −0.5099 0.0000E+00−9.5023E−06 −1.4561E−08 4.5904E−11 −7.7887E−14

The value related to formulae (1) is given as follows: f1/f=−1.690

The value related to formulae (2) is determined as follows: d/f=1.096

Embodiment 6

Focal Length 14.0 mm F num. F/1.0 Angle of Field 2ω = 29.0° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 11.7536 2.0000 Germanium 2 (ASPH) 9.44793.6436 3 (STOP) 8.6541 4 (ASPH) −251.7730 9.0000 Germanium 5 (ASPH)−34.4485 17.1821

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 −4.1759 0.0000E+00  3.5412E−04 −2.4250E−06 −7.2097E−09 3.2997E−11 2 −3.6564 0.0000E+00  6.4071E−04 −5.1496E−06 −2.6194E−08 2.8147E−10 4 −296.2005 0.0000E+00−2.3498E−05 −2.2624E−08   4.2089E−10 1.6620E−13 5 −0.0485 0.0000E+00−1.3375E−05   1.3024E−08 −6.4911E−11 5.7622E−13

The value related to formulae (1) is given as follows: f1/f=−3.270

The value related to formulae (2) is determined as follows: d/f=0.878

Embodiment 7

Focal Length 17.4 mm F num. F/1.0 Angle of Field 2ω = 25.0° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 14.2592 3.4862 Germanium 2 (ASPH) 10.97233.5256 3 (STOP) 8.5719 4 (ASPH) −315.8400 6.5000 Germanium 5 (ASPH)−38.1065 17.5788

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 −5.4326 0.0000E+00  2.6262E−04 −2.2230E−06   1.3209E−08 −6.4906E−11 2 −4.6588 0.0000E+00  5.2710E−04 −5.7051E−06   3.9613E−08 −2.4558E−10 4 −1761.20650.0000E+00 −2.6279E−05   3.6214E−08 −2.7175E−10   5.4799E−13 5 0.55060.0000E+00 −1.4342E−05   1.6966E−09 −6.8841E−11   6.1109E−14

The value related to formulae (1) is given as follows: f1/f=−4.450

The value related to formulae (2) is determined as follows: d/f=0.695

Embodiment 8

Focal Length 18.0 mm F num. F/1.0 Angle of Field 2ω = 24.2° CurvatureDistance between Adjacent Lens Surface # of Radius Lens Pieces/LensThickness Material 1 (ASPH) 14.1777 3.3541 Germanium 2 (ASPH) 10.95263.6873 3 (STOP) 9.7003 4 (ASPH) −263.0890 6.5000 Germanium 5 (ASPH)−38.9490 18.8436

Coefficients, A, B, C, D and E, for the aspheric surfaces as expressedby the formula take their respective values as follows:

Surface # 0 (EP) 2 (A) 4 (B) 6 (C) 8 (D) 10 (E) 1 −5.3215 0.0000E+00  2.6462E−04 −2.2148E−06   1.2973E−08 −5.6049E−11 2 −4.6137 0.0000E+00  5.2576E−04 −5.7043E−06   4.1097E−08 −2.2002E−10 4 −1100.97340.0000E+00 −2.6769E−05   3.4804E−08 −3.0465E−10   6.7690E−13 5 0.45670.0000E+00 −1.4223E−05 −4.1783E−10 −7.5520E−11   9.8844E−14

The value related to formulae (1) is given as follows: f1/f=−4.050

The value related to formulae (2) is determined as follows: d/f=0.743

What is claimed is:
 1. An infrared fixed-focus lens, comprising thefirst lens piece disposed closer to an object and of negativerefractivity, and the second lens piece disposed closer to the imageplane and of positive refractivity, both the first and second lenspieces being made of germanium.
 2. The infrared fixed-focus lensaccording to claim 1, wherein the full angle of field ranges from 24 to55 degrees.
 3. The infrared fixed-focus lens according to claim 1,wherein the first lens piece has its front surface closer to the objectshaped in convexity and exhibits negative refractive power, and thesecond lens piece has its rear surface closer to the image plane shapedin convexity and exhibits positive refractive power.
 4. The infraredfixed-focus lens according to claim 1, wherein the lens meets therequirements as defined in the following formulae (1):−4.5≦f1/f≦−1.55  (1) where f1 is a focal length of the first lens piece,and f is a focal length of the entire optics.
 5. The infraredfixed-focus lens according to claim 1, wherein the lens meets therequirements as defined in the following formulae (2):0.6≦d/f≦1.9  (2) where d is a distance from the first lens piece to thesecond lens piece.